1. Field of the Invention
The present invention relates to a method and apparatus for reducing false contour in a digital display apparatus including a plasma display panel using pulse number modulation.
2. Description of the Related Art
With the development of large display apparatuses along with the commencement of high-definition television (HDTV) broadcast, super-thin and large display apparatuses such as plasma display panel (PDP) displays and digital micromirror device (DMD) displays have been spotlighted. Unlike a cathode-ray tube (CRT) using a current driving method, such matrix display panels display a gray level using pulse number modulation. One TV field is divided into a plurality of subfields in a time domain, and a gray level is displayed using combinations of brightness values of the individual subfields, which are controlled based on the number of sustain pulses during a sustain period for each subfield. In such methods of displaying a gray level using pulse number modulation, an emission position of each subfield inevitably changes depending on an input gray level in the time domain. Although a gray level for still images can be displayed without distortion, false contour not existing on an original image occurs in moving images since an emission position of each subfield significantly changes at even a slight change in an input gray level. In other words, emission pattern change in the time domain is spatially expressed, thereby provoking false contour.
FIG. 1 illustrates an illuminating method used in PDPs. The horizontal axis indicates time, and the vertical axis indicates the number of horizontal scan lines. One field is divided into a plurality of subfields, and each subfield is divided into an address period and a sustain period. During the sustain period, a PDP cell is discharged using a sustain pulse so that the sustain period is maintained for a period of time corresponding to a luminance weight depending on a gray level of an input image, and the gray level of image information is displayed by selectively combining the subfields.
FIG. 2 shows an example of occurrence of false contour. One frame is composed of 8 subfields, and subfields have a luminance weight ratio of 1:2:4:8:16:32:64:128 and gray levels of 127 (pixel A) and 128 (pixel B). When a human retina moves to the right by one pixel in parallel during one field period, a gray level integrated on the human retina is expressed through the integration of subfields in the time direction. Accordingly, when there is a great difference in the emission pattern of a subfield at the same position due to, for example, a motion in a moving image, a gray level having a completely different brightness value than the brightness value of an original input pixel is spatially perceived by the retina, thereby provoking false contour.
In order to solve the problem of false contour, there have been proposed selected combination of subfields for minimizing emission pattern transition associated with a large luminance weight, methods of inserting an equalizing pulse at a position where occurrence of false contour is predicted, and methods of scattering false contour.
In the selected combination of subfields (disclosed in U.S. Pat. Nos. 6,268,890 B1 and 6,310,588 B1), subfields are arranged in substantially increasing or decreasing order of luminance weights, and a subfield combination minimizing the number of subfields with large luminance weights that are “on” is selected out of subfield combinations for which displaying a gray level is possible, thereby reducing occurrences of false contour. In this method, a change in “on/off” subfield diffusion is temporally dispersed, thereby reducing occurrences of false contour. However, since illuminating pattern transition of subfields with relatively large luminance weights is not completely eliminated, false contour cannot be efficiently eliminated. In addition, a large motion causes an error to be large, and thus noise is easily perceived due to error diffusion.
In a method using an equalizing pulse (disclosed in U.S. Pat. No. 6,097,368), the transition between subfields that may cause false contour is detected, and an equalizing pulse is inserted before the transition occurs. In order to obtain an accurate equalizing pulse, an elaborate motion estimator is required. Accordingly, this method is difficult to practically use. In order to overcome this problem, a plurality of optimal equalizing pulse codes are calculated with respect to a current brightness value off line and then stored, and an optimal equalizing pulse code minimizing false contour is selected using the brightness values of corresponding two pixels between current and previous fields. However, there is a limit to efficiently eliminating false contour.
In a method of scattering false contour (disclosed in U.S. Pat. No. 6,088,012), subfields with relatively higher luminance weights are divided into smaller subfields having divided weights, and the smaller subfields are scattered in a field. However, since the higher luminance weights having a large time interval is used to display high gray levels in a moving image, blurring occurs in moving images.